A method is described for the serotyping and subgrouping of rotaviruses by enzyme-linked immuno-sorbent assay (ELISA). For the isolates tested "blind" under code in parallel the serotyping results obtained by ELISA and serum neutralization of fluorescent focus formation were the same. After absorption of the typing antisera with purified heterotypic rotavirus, up to 128-fold differences in titres between isolates were observed in the ELISA test. The results of serotyping and subgrouping, by ELISA, of strains previously described and characterized by the authors' and other laboratories are also given. An attempt is made to correlate the serological findings with variations between virus strains based upon patterns of genome molecular weights as revealed by polyacrylamide gel electrophoresis. The application of this ELISA technique to epidemiological studies and vaccine research is discussed.